In order to meet the emission level requirements, for industrial low emission gas turbine engines, staged combustion is required in order to minimise the quantity of the oxides of nitrogen (NOx) produced. Currently the emission level requirement is for less than 25 volumetric parts per million of NOx for an industrial gas turbine exhaust. The fundamental way to reduce emissions of nitrogen oxides is to reduce the combustion reaction temperature, and this requires premixing of the fuel and all the combustion air before combustion takes place. The oxides of nitrogen (NOx) are commonly reduced by a method which uses two stages of fuel injection. Our UK patent no. 1489339 discloses two stages of fuel injection to reduce NOx. Our International patent application no. WO92/07221 discloses two and three stages of fuel injection. In staged combustion, all the stages of combustion seek to provide lean combustion and hence the low combustion temperatures required to minimise NOx. The term lean combustion means combustion of fuel in air where the fuel to air ratio is low i.e. less than the stoichiometric ratio. In order to achieve the required low emissions of NOx and CO it is essential to mix the fuel and air uniformly so that it has less than a 3.0% variation from the mean concentration before the combustion takes place.
The industrial gas turbine engine disclosed in our International patent application no. WO92/07221 uses a plurality of tubular combustion chambers, whose longitudinal axes are arranged in generally radial directions. The inlets of the tubular combustion chambers are at their radially outer ends, and transition ducts connect the outlets of the tubular combustion chambers with a row of nozzle guide vanes to discharge the hot exhaust gases axially into the turbine sections of the gas turbine engine. Each of the tubular combustion chambers has an annular secondary fuel and air mixing duct which surrounds the primary combustion zone. A plurality of equi-spaced secondary fuel injectors are arranged to inject fuel into the upstream end of the annular secondary fuel and air mixing duct. The annular secondary fuel and air mixing duct has a plurality of equi-spaced outlet apertures to direct the fuel and air mixture into the secondary combustion zone. Each of the tubular combustion chambers of the three stage variant also has an annular tertiary fuel and air mixing duct which surrounds the secondary combustion zone. A plurality of equi-spaced tertiary fuel injectors are arranged to inject fuel into the upstream end of the annular tertiary fuel and air mixing duct. The annular tertiary fuel and air mixing duct has a plurality of outlet apertures to direct the fuel and air mixture into the tertiary fuel and air mixing zone.
Unfortunately the flow of air into the tubular combustion chambers is not uniform, this is because of an asymmetric flow of air from a diffuser at the downstream end of the gas turbine engine compressor to the tubular combustion chambers. Each of the secondary fuel injectors passes identical fuel flows and therefore a non uniform fuel and air mixture is created at the points of injection due to the non uniform air flow. The fuel and air mixture directed from the outlet apertures into the secondary combustion zone is non uniform. Similarly the fuel and air mixture directed from the outlet apertures of the tertiary mixing duct into the tertiary combustion zone will be non uniform. This increases the emissions of NOx to above the acceptable levels.
An initial solution for the problem was to redistribute the fuel to match the air mass flow distribution by adjusting the fuel hole sizes of the individual fuel injectors. This requires all of the fuel injectors to be unique in fuel hole diameters and position of the fuel holes to match the air mass flow to achieve the required uniformity of mixing. The air mass flow distribution also varies with the operating power range of the engine. However redistributing the fuel to match the air mass flow distribution would not achieve the required 3.0% variation in concentration uniformity at all powers and hence emissions of NOx would be above the acceptable levels.
Another solution for the problem was to fit air guidance devices upstream of the secondary fuel and air mixing duct, and tertiary fuel and air mixing duct, to create a uniform air mass flow at the intakes of the secondary fuel and air mixing duct, and tertiary fuel and air mixing duct. Unfortunately any minor changes in the air guidance devices formed during the production processes result in relatively large changes in air mass flow distribution i.e. greater than the 3.0% variation in concentration uniformity.
A further solution for the problem was to redistribute the air mass flow upstream of the intakes of the secondary fuel and air mixing duct, and tertiary fuel and air mixing duct, using a flow distributor which uses its pressure drop to create uniform flow through each of its flow routes. Unfortunately an increase in system pressure drop is not acceptable because this reduces the surge margin of the compressor and also reduces the thermal efficiency of the engine i.e. increases the engine fuel consumption.
The only acceptable solution therefore must be tolerant to upstream air flow variations without increasing the system pressure loss.
EPO388886A discloses a combustor for burning of fuel by premixing fuel with air in a number of premix flame forming nozzles which inject the premixed fuel and air into a secondary combustion zone. Fuel injectors are provided to inject fuel into the premix flame forming nozzles downstream of the intakes of the premix flame forming nozzles.
The present invention seeks to provide a novel gas turbine engine combustion chamber which overcomes the above mentioned problem.
Accordingly the present invention provides a gas a turbine engine combustion chamber comprising a primary combustion zone defined by at least one peripheral wall and an upstream end wall connected to the upstream end of the at least one peripheral wall, the upstream end wall has at least one aperture, primary air intake means and primary fuel injector means to supply air and fuel respectively through the at least one aperture into the primary combustion zone, a secondary combustion zone in the interior of the combustion chamber downstream of the primary combustion zone, means to define a plurality of secondary fuel and air mixing ducts, each secondary fuel and air mixing duct has an outlet at its downstream end for discharging the fuel and air mixture into the secondary combustion zone, each secondary fuel and air mixing duct has secondary air intake means at its upstream end to supply air into the secondary fuel and air mixing duct, each secondary fuel and air mixing duct has secondary fuel injector means arranged to supply fuel into the secondary fuel and air mixing duct, each secondary fuel injector means is located downstream of the secondary air intake means of the associated secondary fuel and air mixing duct, the outlets of the secondary fuel and air mixing ducts have substantially equal flow areas to produce substantially the same air flow rate through each of the secondary fuel and air mixing ducts, the secondary fuel injector means of each secondary fuel and air mixing duct is arranged to supply substantially the same flow rate of fuel so that the fuel to air ratio of the mixture leaving each of the secondary fuel and air mixing ducts is substantially the same.
Preferably the secondary fuel and air mixing ducts radially inwardly of the primary combustion zone, the secondary fuel and air mixing ducts are defined at their radially inner extremity and radially outer extremity by a second pair of walls and a plurality of walls extending radially between the second pair of annular walls.
Preferably at least one of the secondary fuel injector means comprises a hollow cylindrical member, the hollow cylindrical member has a plurality of apertures spaced apart axially along the cylindrical member to inject fuel into the secondary fuel and air mixing duct.
The hollow cylindrical member may extend axially with respect to the axis of the combustion chamber. The hollow cylindrical member may extend radially with respect to the axis of the combustion chamber. The apertures in the hollow cylindrical member may be arranged to direct the fuel circumferentially.
Preferably the walls extending radially between the annular walls are secured to both the annular walls.
Preferably the secondary fuel injector means for at least one of the secondary fuel and air mixing ducts comprises two secondary fuel injectors. The two secondary fuel injectors may be spaced apart circumferentially relative to the axis of the combustion chamber. Preferably each secondary fuel injector is arranged to supply fuel to the upstream end of the associated secondary fuel and air mixing duct.
Preferably the combustion chamber includes means to define a plurality of tertiary fuel and air mixing ducts, each tertiary fuel and air mixing duct is in fluid communication at its downstream end with a tertiary combustion zone in the interior of the combustion chamber downstream of the secondary combustion zone, each tertiary fuel and air mixing duct has tertiary air intake means at its upstream end to supply air into the tertiary fuel and air mixing duct, each tertiary fuel and air mixing duct has tertiary fuel injector means arranged to inject fuel into the tertiary fuel and air mixing duct, the tertiary fuel and air mixing ducts are arranged in an annulus outside the peripheral wall, each tertiary fuel injector means is located downstream of the tertiary air intake means of the associated tertiary fuel and air mixing duct, each tertiary fuel and air mixing duct has an outlet at its downstream end for discharging the fuel and air mixture into the tertiary combustion zone, the outlets of the tertiary fuel and air mixing ducts have substantially equal flow areas to produce substantially the same air flow rate through each of the tertiary fuel and air mixing ducts, the tertiary fuel injector means of each tertiary fuel and air mixing duct is arranged to supply substantially the same flow rate of fuel so that the fuel to air ratio of the mixture leaving each of the tertiary fuel and air mixing ducts is substantially the same.
Preferably the tertiary fuel and air mixing ducts are defined by a radially inner annular wall, a radially outer annular wall and a plurality of walls extending radially between the pair of annular walls, the radially extending walls are secured to at least one of the pair of annular walls.
Preferably the tertiary fuel and air mixing ducts are arranged around the combustion chamber.
The combustion chamber may be tubular, the peripheral wall of the primary combustion zone is annular and the upstream end wall has a single aperture, the plurality of tertiary fuel and air mixing ducts are arranged circumferentially in an annulus radially outwardly of the secondary combustion zone.
Preferably at least one of the tertiary fuel injector means comprises a hollow cylindrical member, the hollow cylindrical member has a plurality of apertures spaced apart axially along the cylindrical member to inject fuel into the tertiary fuel and air mixing duct.
The hollow cylindrical member may extend axially with respect to the axis of the combustion chamber. The hollow cylindrical member may extend radially with respect to the axis of the combustion chamber. The apertures in the hollow cylindrical member may be arranged to direct the fuel circumferentially.
Preferably the tertiary fuel injector means for at least one of the tertiary fuel and air mixing ducts comprises two tertiary fuel injectors. The two tertiary fuel injectors may be spaced apart axially relative to the axis of the combustion chamber. The two tertiary fuel injectors may be spaced apart circumferentially relative to the axis of the combustion chamber.
The present invention also provides a gas turbine engine combustion chamber comprising a primary combustion zone defined by at least one peripheral wall and an upstream end wall connected to the upstream end of the at least one peripheral wall, the upstream end wall has at least one aperture, primary air intake means and primary fuel injector means to supply air and fuel respectively through the at least one aperture into the primary combustion zone, a secondary combustion zone defined by a downstream portion of the at least one peripheral wall, the secondary combustion zone is in the interior of the combustion chamber downstream of the primary combustion zone, secondary air intake means and secondary fuel injector means to supply air and fuel respectively into the secondary combustion zone, means to define a plurality of tertiary fuel and air mixing ducts, each tertiary fuel and air mixing duct is in fluid flow communication at its downstream end with a tertiary combustion zone in the interior of the combustion chamber downstream of the secondary combustion zone, each tertiary fuel and air mixing duct has tertiary air intake means at its upstream end to supply air into the tertiary fuel and air mixing duct, each tertiary fuel and air mixing duct has tertiary fuel injector means arranged to supply fuel into the tertiary fuel and air mixing duct, each tertiary fuel injector means is located downstream of the tertiary air intake means of the associated tertiary fuel and air mixing duct, each tertiary fuel and air mixing duct has an outlet at its downstream end for discharging the fuel and air mixture into the tertiary combustion zone, the outlets of the tertiary fuel and air mixing ducts have substantially equal flow areas to produce substantially the same air flow rate through each of the tertiary fuel and air mixing ducts, the tertiary fuel injector means of each fuel and air mixing duct is arranged to supply substantially the same flow rate of fuel so that the fuel to air ratio of the mixture leaving each of the tertiary fuel and air mixing ducts is substantially the same.
Preferably the tertiary fuel and air mixing ducts are arranged around the combustion chamber.
Preferably the tertiary fuel and air mixing ducts are arranged in an annulus outside the peripheral wall, the tertiary fuel and air mixing ducts are defined by a radially inner annular wall, a radially outer annular wall and a plurality of walls extending radially between the pair of annular walls, the radially extending walls are secured to at least one of the pair of annular walls.